Optical disks such as compact disks (CDs), video compact disks (VCDs) and digital versatile disk (DVDs) are able to be played by recording and reproducing apparatuses. When an optical pickup head of an optical disk drive operates, the light emitted by a light source such as a laser diode is focused by an object lens of the optical pickup head on an optical disk, and the light reflected by the optical disk is transmitted to a light sensor to realize information from the disk.
Referring to FIG. 1, the optical pickup head 10 moves along two main directions, i.e. a direction perpendicular to the disc face, referred as a focusing direction F, and a direction parallel to the disc face, referred as a following direction T.
During operation, a focusing error and a tracking error usually arise on the rotating disk. The focusing error is generally caused by vibration of the disk in the focusing direction F, and the tracking error is caused by eccentricity of the disk in the following direction T. To correct these errors, a tracking control system for an optical pickup head was developed, as can be seen in FIG. 2. The optical pickup head 20 has six light receiving parts A, B, C, D, E and F for producing output signals, when receiving light reflected from the optical disk. The output signals of E and F parts are amplified through a pre-amplifier 21 to produce a tracking error signal TE. The tracking error signal TE is processed by a lead-lag compensating apparatus 23 included in a digital signal processor (DSP). In the lead-lag compensating apparatus 23, either a first lead-lag compensator 231 or a second lead-lag compensator 232 is selectively used to generate a tracking output signal TRO. The tracking output signal TRO is transmitted to an actuator 24 to provide a moving force for actuating a trace shift of the optical pickup head 20 in the following direction accordingly.
The first lead-lag compensator 231 comprises a first lead compensator 2311 and a first lag compensator 2312, and the second lead-lag compensator 232 comprises a second lead compensator 2321 and a second lag compensator 2322. The first lead-lag compensator 231 and the second lead-lag compensator 232 have respective gains for complying with various operational statuses. For example, the first lead-lag compensator 231 with a relatively low gain is selected when the optical pickup head 20 is in a following mode. In contrast, the second lead-lag compensator 232 with a relative high gain is selected when the optical pickup head 20 is in a seeking mode. The term “following mode” used herein means that the position of the optical pickup head with respect to the tracks is maintained in the proper center position above the selected track. The term “seeking mode” means that the optical pickup head jumps from one track to another.
In particular, when the optical pickup head 20 is switched from a following mode to a seeking mode, the work for compensation is shifted from the first lead-lag compensator 231 to the second lead-lag compensator 232. Meanwhile, the second lead-lag compensator 232 is operated under preset initial values to provide an initial kick force of the actuator 24 to move the optical pickup head for seeking operation. During the seeking mode, a velocity error signal Verr is produced by a velocity profile generator 25 according to a frequency variation level of the tracking error signal TE, which is indicative of the velocity of the optical pickup head relative to the velocity of the disk. The velocity error signal Verr is then sent into the second lead-lag compensator 232 to adjust the kick force for controlling movement of the actuator 24.
FIG. 3(a) schematically illustrates some positions of the optical pickup head 20 occurring relative to the track 1 of an optical disk in a following mode. Due to the influence of inherent eccentric feature of the optical disk, which is particularly significant under high revolving speed, the optical pickup head 20 may swing inwardly or outwardly like a sine wave relative to the track such that the positions thereof are slightly deviated from the ideal trajectory. Therefore, the tracking error signal TE and the tracking output signal TRO also swing in similar sine-wave manners, as can be seen in FIGS. 3(b) and 3(c), respectively. That is to say, the optical pickup head 20 has different momentum at different positions upon switching from the following mode to the seeking mode. For example, the optical pickup head 20 swinging to position 2 tends to move toward the track 2. In contrast, the optical pickup head 20 swinging to position 4 tends to move toward the track 0. Therefore, at the moment the following mode is switched to the seeking mode, the energy required for moving the optical pickup head from different positions around a starting track to the target track will be different. This effect is particularly significant when the seeking operation is a short seeking one. Since the initial value provided for the second lead-lag compensator 232 is constant in the prior art at the moment the optical pickup head is switched from the following mode to the seeking mode, the above-mentioned effect resulting from varied positions of the optical pickup head around the starting track is not considered by the prior art compensator 232. Therefore, when the optical pickup head 20 jumps only a few tracks, for example less than ten tracks, seeking failure may occur due to the improper initial kick force, which results in inferior data pickup performance.